1. Technical Field
This invention relates to a magneto-optical recording medium wherein the signals reproduced are amplified in the reproduction through enlargement of the recorded magnetic domains.
2. Background Art
A magneto-optical recording medium is a recording medium wherein temperature of the magnetic thin film is locally elevated by means of a laser beam or the like to reduce coercive force, and wherein the direction of magnetization in this part is reversed with external magnetic field, or alternatively, the magnetization direction of initialization is maintained to thereby form a magnetic domain (record mark), and wherein the magnetization direction of the magnetic domain is read out by utilizing Kerr effect or Faraday effect.
In conventional magneto-optical recording media, density of the reproducible magnetic domains has been limited by the diameter of the laser beam spot used for the reproduction, and the magnetic domains of a diameter less than half of the laser beam spot diameter has been hardly reproducible.
Japanese Patent Application Kokai No. (JP-A) 8-7350 discloses an exemplary magneto-optical recording medium wherein production of the magnetic domain with a diameter less than half of the laser beam spot diameter has been enabled. This magneto-optical recording medium has a recording film comprising three magnetic layers, namely, first, second and third magnetic layers disposed on the substrate in this order, and the three magnetic layers are exchange-coupled. In the reproduction, the medium is irradiated with a laser beam from the side of the substrate, and reproduction magnetic field is applied from the side of the recording film. The recorded magnetic domain is retained in the third magnetic layer, and the magnetic domain is copied to the first and the second magnetic layers by laser beam irradiation. The thus copied magnetic domains enlarge in in-plane directions of the magnetic layers upon application of the read magnetic field. The copied, enlarged magnetic domain is read out as in the case of conventional magneto-optical recording media. When the reproduction of the copied, enlarged magnetic domain is completed, the copied magnetic domain is erased by applying erasing magnetic field whose direction is opposite to the reading magnetic field. By repeating such procedure, small magnetic domains which had been unreproducible in conventional method could be reproduced. The merit of this process is not only the high reproduction resolution. Since the magnetic domains are actually enlarged, fundamental enhancement in the intensity of the reproduced signals is enabled.
In the aforesaid JP-A 8-7350, the second magnetic layer comprises a rare earth and metal-transition metal alloy which has a compensation temperature higher than room temperature, and the second magnetic layer has a Curie temperature which is lower than the recording temperature of the minute magnetic domain. The third magnetic layer also comprises a rare earth and metal-transition metal alloy, and at room temperature, the magnetic moment as a whole is in the same direction as the magnetic moment of the rare earth metal.
The inventors of the present invention, however, have found that the intensity of the signals reproduced is only insufficiently improved by the recording medium having the recording film constitution as described in the aforesaid JP-A 8-7350 since the exchange coupling force between the magnetic layers is too strong. In addition, the aforesaid JP-A 8-7350 requires fabrication of the second magnetic layer from a rare earth metal-transition metal alloy having a compensation temperature higher than room temperature. The compensation temperature, however, greatly fluctuates by slight difference in the composition, and it is extremely difficult to adjust the compensation temperature to a particular temperature range above the room temperature.